Carbon containing layer

ABSTRACT

A workpiece comprises a substrate having a sliding surface provided with a friction-reducing coating. Such coating consists essentially of elemental carbon dispersed in a matrix formed of at least one metallic element in the proportions of 50.1 to 99.1 at % of the elemental carbon and 0.1 to 49.9 at % of the metallic element. The ratio of the metallic element to the elemental carbon differs from the stoichiometric ratio of the carbide.

.Iadd.This is a continuation of application Ser. No. 338,438, filed Apr.13, 1989, now abandoned, which was in turn a continuation of Ser. No.051,989 filed May 19, 1987, now abandoned which was an application toreissue U.S. Pat. No. 4,525,417 issued Jun. 25, 1985 from Ser. No.469,363 filed Feb. 24, 1983. .Iaddend.

This invention relates to a carbon-containing sliding layer or coatingbetween surfaces moving against each other or on each other (slidingsurfaces) as well as to methods of manufacturing such layers.

In order to reduce the frictional resistance of fixed sliding surfaces,lubricants are used which reduce detrition of material and energyconsumption.

In the friction of fixed sliding surfaces the frictional force ortangential force K depends on the coefficient of sliding friction μ ofthe sliding surfaces and the load P with which the sliding surfaces rubon each other or against each other:

    K=μP.

A sliding surface is to be understood to mean a surface of a workpiecewhich slides on an opposing surface of another workpiece under a givenpressure force.

The coefficient of sliding friction μ depends on the roughness of therubbing surfaces, in the case of smooth surfaces, however, on thecombination of materials, thus on the materials of the sliding surfaces,as well as on the ambient atmosphere.

It is known to vary the coefficient of sliding friction of dry frictionby coating the sliding surfaces with different substances so as toreduce the friction and to reduce the detrition of the rubbing surfaces.

On the basis of their material properties, the known solid lubricantsused for this purpose form different groups:

1. Substances which on the basis of a layered crystal structure havegood sliding friction properties, for example, graphite or thechalcogenides of metals such as molybdenum, tungsten and niobium;

2. Ductile metals, for example, gold, silver, lead or tin;

3. Materials of greater hardness, for example, borides, silicides,nitrides or, carbides.

Such materials may be provided loose between the sliding surfaces or befixed on the sliding surfaces in the form of thin layers by means ofmethods commonly used in thin layer technology, for example, vapourdeposition in a vacuum or cathode sputtering.

All these solid lubricants have their special advantages and theirdisadvantages.

A particular disadvantage of graphite as a solid lubricant is thatalthough under normal atmospheric conditions it has a low coefficient ofsliding friction (μ≈0.1 to 0.2), which is desired, in a dry atmosphereit shows a considerably increased coefficient of sliding friction (μ≈0.8). The result is that the detrition of the rubbing surfaces in a dryatmosphere is great, since graphite is a very soft material, i.e. islittle wear-resistant, and moreover the friction is increased by theincreased coefficient of sliding friction of graphite.

Friction problems at temperatures which are not very high can readily besolved by the use of chalcogenides of the indicated kind, but the use ofthese materials presents problems at higher temperatures. MoS₂decomposes in air at temperatures above 400° C. Such chemicalinstability hence restricts the use of materials which in principlewould readily be suitable as solid lubricants. Moreover, MoS₂, incontrast with graphite, has a very low coefficient of sliding friction(μ≈0.04) in a dry atmosphere, but under normal atmosphere conditions thecoefficient of sliding friction μ increases to approximately 0.2.

A general disadvantage of materials having a layered structure--whichapplies to both graphite and to MoS₂ --is moreover their insufficientwear resistance as well .Iadd.as .Iaddend.their low hardness.

Ductile metals, for example, gold, silver, lead and tin, are good solidlubricants since these materials have comparatively low coefficients ofsliding friction (μ≈0.2 to 0.4). However, since these metals are verysoft--which property on the other hand gives them their good slidingproperties--they, too, have a low wear resistance.

In order to obtain layers of higher wear resistance, materials ofgreater hardness, for example, silicides, borides, nitrides andcarbides, have been used on sliding surfaces.

It has been found that, although with these materials of greaterhardness a good wear resistance can be achieved, the coefficients ofsliding friction have values (μ≈0.3 to 0.7) considerably above thevalues of the materials which can readily be used as solid lubricantsdue to their low coefficients of sliding friction, graphite (μ≈0.1 to0.2) or molybdenum sulphide MoS₂ (μ≈0.04).

Of particularly great technical importance are frictionally engagedcombinations in which steel parts slide on each other without or withinsufficient hydrodynamic lubrication. With such friction combinationsthere is desired as a rule a very long life, i.e. low detrition, as wellas a small friction which is independent of ambient conditions.

A carbon layer having a diamond-like crystal structure as a frictionmember against steel is known from published European application22,285, which under vacuum or inert gas conditions has a particularlylow coefficient of sliding friction μ and at the same time a high wearresistance and hardness; however, these carbon layers have thedisadvantage that they do not have a low coefficient of sliding frictionwhich is substantially independent of the relative air humidity.

It is therefore the object of the present invention to improve acarbon-containing sliding layer which has the function of a solidlubricant in such manner that it exhibits a low coefficient of slidingfriction which is independent of the relative air humidity and moreovermaintains the favourable properties of the known carbon layer, namelyreadily adheres in particular to steel, is wear resistant and hard andhas a low coefficient of sliding friction against steel.

According to the invention this object is achieved in that the slidinglayer comprises 50.1 to 99.9 at % of elemental carbon and 0.1 to 49.9 at% of at least one metallic element in a ratio which does not correspondto the stoichiometric ratio of a carbide.

According to advantageous modified embodiments of the invention thesliding layer comprises 60 to 97 in particular 80 to 95 at % ofelemental carbon and 3 to 40, in particular 5 to 20 at % of at least onemetallic element.

According to advantageous modified embodiments of the invention themetallic element is an element of group Ib, IIb, IIIa, IV, Vb, VIb, VIIbor VIII of the periodic table. Included are copper (Cu), silver (Ag),gold (Au), zinc (Zn), cadmium (Cd), boron (B), aluminium (Al), gallium(Ga), indium (In), thallium (Tl), silicon (Si), titanium (Ti), germanium(Ge), zirconium (Zr), tin (Sn), hafnium (Hf), lead (Pb), vanadium (V),niobium (Nb), tantalum (Ta), chromium (Cr), molybdenum (Mo), tungsten(W), manganese (Mn), rhenium (Re), iron (Fe), ruthenium (Ru), osmium(Os), cobalt (Co), rhodium (Rh), iridium (Ir), nickel (Ni), palladium(Pd) and platinum (Pt).

According to a further particularly advantageous embodiment of theinvention the metallic element is tantalum, ruthenium, tungsten, iron orsilicon.

A method of manufacturing said sliding layer is characterized in that itis deposited by chemical or physical vapour deposition on a substrate tobe coated, in which in particular carbon and the selected metallicelement(s) are deposited by means of cathode sputtering in an inert gasatmosphere.

Carbon and the selected metallic element(s) are deposited in particularby sputtering of a target formed from the selected metallic element(s)in an atmosphere of an inert gas and a hydrocarbon gas.

According to a particularly favourable embodiment of the method thedeposition process is first started in an atmosphere which contains onlyan inert gas and the deposition process is then continued under anatmosphere of an inert gas--hydrocarbon gas. The advantage is that firsta layer is deposited which consists only of the selected metallicelement(s) which layer improves the adhesion and the actual slidinglayer which comprises carbon in addition to the metallic elements isthen deposited.

The invention is based on the recognition of the fact that theproperties of the known carbon layer having a diamond-like structurewhich is good in itself as regards the friction ratio and adhesion tosteel, in particular can still be improved when the carbon forming thelayer is deposited or dispersed in elemental form in a matrix ofmetallic elements.

This matrix improves the hardness and decisively improves the adhesionof the carbon layer to the substrate. Within the scope of the inventiona variety of metallic elements may be used for constructing a metalmatrix in which the carbon is quasi incorporated, provided they can beapplied to a substrate by means of chemical or physical vapourdeposition. It is not necessary at all that the metallic element shouldbe a known ductile metal described above for solid lubrication purposes.

Carbon alone shows comparatively different values for the slidingfriction coefficient, for example, against steel in accordance with therelative humidity of the ambient atmosphere. The specifichumidity-independent friction ratio of the layers according to theinvention with respect to a friction member is determined in particularby the choice of the matallic element from the indicated groups of theperiodic table as a matrix for the carbon.

Moreover this is decisive as to the adhesion of the layers to therequired substrate, which is very essential for the use of mechanicallystressed layers.

It is to be emphasized that the portion of the metallic element in thelayer may at any rate not be greater than 49.9 at % since otherwise thedesired properties of the sliding layer deteriorate decisively.

It has been found that substantially all metallic elements are suitablefor use as a matrix in which the carbon of the sliding layer isincorporated in elemental form.

It is surprising that the metal matrix in combination with the carbonproduces such excellent sliding layers of .Iadd.low .Iaddend.coefficientof sliding friction.

On the basis of the material properties of the individual components ofsaid layer compositions said effect would not be expected at all in eachcase; the coefficients of sliding friction of steel members, including,for example, iron, are in the range from μ≈0.6 to ≈1 in a normalatmosphere. Iron would hence be a completely unsuitable material to forma lubricating layer against steel.

However, in combination with carbon preferably lower coefficients ofsliding friction are produced with μ≈0.14 in a normal atmosphere (50%relative air humidity).

The advantages which can be obtained by means of the invention consistin particular in that the sliding layers according to the invention arenot only hard and wear-resistant and readily adhere to the substrate,but they also have a low sliding friction coefficient against steelwhich is substantially independent of the relative air humidity. Afurther advantage to be considered is that the sliding layers accordingto the invention can also reduce the detrition of tool surfaces due totheir wear-resistance, so that they may be used as coating layersparticularly for steel tools.

A particular advantage of the method of manufacturing the sliding layersaccording to the invention is to be considered in that, in aparticularly simple manner and without interrupting the process, first alayer favouring the adhesion of the sliding layer can be deposited fromthe metallic element forming the matrix; just with respect to steelsubstrates, a very readily adhering optimum intermediate layer of ametallic element is obtained in this manner between the steel substrateand the actual sliding layer consisting of elemental carbon and at leastone metallic element.

Within the scope of the invention, metallic elements are to beunderstood in a narrower sense to be those chemical elements which arelight-impervious only in the solid state, show lustre as a result of ahigh light reflection power, are good current and heat conductors, canbe plastically deformed by rolling, drawing, pressing, cutting and thelike, and have atomic lattices. In a broader sense, however, themetallic elements are to be understood to include also the chemicalelements which are not nonmetals and have pronounced metalliccharacteristics, for example, the elements sometimes referred to assemimetals and sometimes as metals such as silicon (compare, forexample, Lueger, Lexikon der Technik, Deutsche Verlagsanstalt Stuttgart,4th Edition 1961, Vol. 3, p. 636).

It is important to point out that the claimed layers do not relate tothermodynamically stable carbides, for example TiC or WC; the describedlayers are as a rule X-ray amorphous and can be manufactured in anydesired composition. The carbon portion required for low friction valueslies considerably above the stoichiometrically exact compositionrequired for a chemical compound--very low coefficients of slidingfriction are often achieved only with 80-95 at %. Moreover, many of theclaimed layer compositions are known as stable carbides (for example,Ru-C). The stable carbides (for example, TiC, WC) show coefficients ofsliding friction against steel as a friction member of μ≈0.2 to 0.4,which are considerably above the values which the claimed layers show.For W-C layers manufactured within the scope of the invention and having91 at % carbon and 9 at % tungsten a coefficient of sliding frictionμ≈0.14 was measured at a relative air humidity of 50% with steel as afriction member, and a .[.cofficient.]. .Iadd.coefficient .Iaddend.ofsliding friction μ≈0.06 as measured with a second W-C layer of the samecomposition as a friction member.

Thus, the layers according to the invention are of great importance aslubricating the protective layers, for example, for sliding bearings andoscillating friction combinations (for example, shaving heads ofelectrical shaving apparatuses) or knives, for example, for cuttingmetal plates.

.Iadd.Brief Description of the Drawing .Iaddend.

The invention will now be described with reference to the accompanyingdrawing, in which the FIGURE is a sectional view of a sliding layerprovided on a substrate with an intermediate layer promoting itsadhesion to the substrate.

1. Manufacture of iron-carbon layers

The FIGURE is a sectional view of a substrate 1, for example, ofchromium-nickel steel, having an adhesion-promoting intermediate layer 3of pure iron and a sliding layer 5 of iron and carbon. These layers canbe obtained, for example, as follows: layers having a thickness of 0.9μm and a micro hardness of 15×10³ N/mm² were manufactured by cathodesputtering of a pure iron target in an atmosphere of an inert gas at apressure of 20 mbar, for example argon, and a hydrocarbon gas at apressure of 0.2 mbar, for example, acetylene. Flat rings of 100 Cr6steel served as a substrate. The layers approximately 20 at % iron, 78at % carbon and the remainder unknown impurities (it is assumed that thelatter derives from the atmosphere of the cathode sputtering process),have an excellent adhesion to their substrate, which is due inter aliato the fact that during the first minutes of the coating process thecathode sputtering process was carried out in a pure inert gasatmosphere so that first a pure iron layer was deposited. The furthercoating then occurred in the above-mentioned atmosphere. The layers thusmanufactured have a sliding friction coefficient in a dry atmosphere(<0.1% relative humidity) of μ≈0.06 and in a moist atmosphere (≈80%relative humidity) of μ≈0.15.

Layers which were manufactured by sputtering of an iron target and aniron carbon target, respectively, in an inert gas-hydrocarbon gasatmosphere, have a hardness in the range from 15×10³ to 30×10³ N/mm²(Knoop hardness). The sliding friction coefficients against steel as afriction member as a rule are in the range from μ≈0.05 to 0.3 dependingon the coating conditions. For example , for a layer which contains<49.9 at % iron as well as >5.01 at % carbon a coefficient of slidingfriction which is far less dependent on the relative air humidity can beachieved than is the case for pure carbon layers. For pure carbon thevalues for the coefficient of sliding friction μ against steel with arelative air humidity of <0.1% are μ≈0.02 and with a relative airhumidity of 95% μ≈0.2.

An iron-carbon layer of a different composition, namely 84 at % carbonand 16 at % iron, was manufactured under the same conditions and with arelative air humidity of 50% had sliding friction coefficients μ≈0.14both against steel and also against a layer of the same composition as afriction member (compare Table 4).

In principle the compositions of the layers with respect to thequantities of carbon and metallic element(s) can be controlled by thetarget composition and by the amount of the hydrocarbon gas,respectively, in the sputtering atmosphere, for example in the sensethat, when the concentration of the carbon in the layer is to be higher,the amount of the hydrocarbon gas in the sputtering atmosphere isincreased accordingly. The exact ratios can be determined by simpleexperiments.

Table 1 illustrates how the coefficient of sliding friction μ foriron-carbon layers containing 3.7 at % iron, 95.5 at % carbon and 0.8 at% residual gas with a friction member in the form of a layer of the samecomposition as well as with steel as a friction member is influenced bydifferent relative air humidities.

                  TABLE 1                                                         ______________________________________                                                       Relative air                                                   Sliding layer  humidity (%) μ.sub.1                                                                           μ.sub.2                                 ______________________________________                                        3.7 at % Fe    90           0.15   0.13                                       +              50           0.14   0.12                                       95.5 at % C    10           0.13   0.11                                       +              1            0.07   0.07                                       0.8 at % residual gas                                                                        <0.4         0.04   0.04                                       ______________________________________                                         μ.sub.1 = coefficient of friction with a friction member in the form o     a layer of the same composition as the sliding layer.                         μ.sub.2 = coefficient of friction against steel as a friction member. 

Further compositions in the range from 65.2 to 95.5 at % carbon and 32.9to 2.3 at % iron with the remainder, consisting of gases incorporated inthe resulting layer in the cathode sputtering process were tested fortheir friction behaviour (compare table 2). The layers were manufacturedunder the same parameters as indicated for the above-describediron-carbon layer. Coefficients of sliding friction μ from ≈0.10 to≈0.17 in an atmosphere with 50% relative air humidity with steel as afriction member and values for μ from ≈0.13 to ≈0.22 against a layer ofthe same composition as a friction member were measured. The layercompositions and the associated coefficients of sliding friction are setforth in Table 2.

                  TABLE 2                                                         ______________________________________                                                           Residual gases                                             Fe (at %) C (at %) (at %)        μ.sub.1                                                                         μ.sub.2                              ______________________________________                                         2.3      93       4.7           0.16 0.16                                     3.7      95.5     0.8           0.14 0.12                                    14.8      78.1     7.1           0.14 0.17                                    20.3      78.7     1.0           0.13 0.15                                    32.9      65.2     1.9           0.22 0.10                                    ______________________________________                                         μ.sub.1 = coefficient of friction with 50% relative air humidity and a     friction member in the form of a layer of the same composition as the         sliding layer.                                                                μ.sub.2 = coefficient of friction with 50% relative air humidity and       steel as a friction member.                                              

2. Manufacture of a tantalum-carbon layer

As in the example of the iron-carbon layer, layers having a thickness of0.9 μm were manufactured by cathode sputtering of a pure tantalum targetin an atmosphere of an inert gas at a pressure of 20 mbar, for example,argon, and a hydrocarbon gas at a pressure of 0.2 to 10 mbar, forexample, acetylene. As a substrate there were used steel rings orsilicon monocrystalline disks.

The layers with 95 at % carbon and 5 at % tantalum in an atmosphere of arelative air humidity of 50% have a coefficient of sliding frictionagainst steel of μ≈0.08 and against a layer of the same composition as afriction member of μ≈0.03 (compare Table 4).

3. Manufacture of a ruthenium-carbon layer

Layers having a thickness of 0.9 μm were manufactured by cathodesputtering of a pure ruthenium target in an atmosphere of an inert gasat a pressure of 20 mbar, for example argon, and a hydrocarbon gas at apressure of 0.2 to 1.0 mbar, for example, acetylene. As substrates therewere used steel rings or silicon monocrystalline disks.

A layer which comprises 18 at % ruthenium at 82 at % carbon has asliding friction coefficient in an atmosphere with 50% relative airhumidity of μ≈0.05 with steel as a friction member and of μ≈0.03 with alayer of the same composition as a friction partner (compare Table 4).

4. Manufacture of tungsten-carbon layers

Layers having a thickness of 0.9 μm and a micro hardness of 21×10³ N/mm²were manufactured by cathode sputtering of a pure tungsten target in anatmosphere of an inert gas at a pressure of 20 mbar, for example, argonand a hydrocarbon gas at a pessure of 0.2 to 1.0 mbar, for example,acetylene. As substrates there were used steel rings or siliconmonocrystalline discs.

A layer containing 9 at % tungsten and 91 at % carbon has a slidingfriction coefficient in an atmosphere with 50% relative air humidity ofμ≈0.14 with steel as a friction member and of μ≈0.06 with a layer of thesame composition as a friction member (compare Table 4).

Further compositions in the range of 66.2 to 96.5 at % carbon and 30.6to 1.6 at % tungsten with the remainder consisting of gases incorporatedin the resulting layer during the cathode sputtering process were testedfor their friction behaviour (compare Table 3). The layers weremanufactured under the same parameters as described above for theiron-carbon layers, tantalum-carbon layers and ruthenium-carbon layers.Coefficients of sliding friction μ from ≈0.10 to 26 0.17 in anatmosphere with 50% relative air humidity with steel as a frictionmember were measured. The layer compositions and the associatedcoefficients of sliding friction are set forth in Table 3.

Table 3 also includes a layer composition having 59.3 at % tungsten,36.7 at % carbon and 4.0 at % residual gas from the cathode sputteringatmosphere; said layer composition is beyond the claimed range ofcompositions. It had a poor adhesion to the substrate, a high detritionin friction tests and coefficient of sliding friction μ≈0.4 with steelas a friction member in an atmosphere with 50% relative air humidity.

The tungsten-carbon layers set forth in Table 3 illustrate that thecoefficient of sliding friction of the sliding layers decreases withincreasing carbon portion.

                  TABLE 3                                                         ______________________________________                                        W              Residual gases                                                 (at %)                                                                              C (at %) (at %)      μ.sub.2                                                                           Remarks                                     ______________________________________                                        1.6   94.5     3.9         0.17                                               2.8   96.5     0.7         0.10                                               4.9   95       0.1         0.13                                               5.3   92.9     1.8         0.16                                               8.9   90.3     0.8         0.14                                               30.6  66.2     3.2         0.14   hardness                                                                      30 × 10.sup.3 N/mm.sup.2              59.3  36.7     4.0         ˜0.4                                                                           very high                                                                     detrition                                   ______________________________________                                         μ.sub.2 = coefficient of friction with 50% relative air humidity and       steel as a friction member.                                              

5. Manufacture of a silicon-carbon layer

Layers having a thickness of 0.9 μm were manufactured by cathodesputtering of a pure silicon target in an atmosphere of an inert gas ata pressure of 20 mbar, for example, argon, and a hydrocarbon gas at apressure of 0.2 to 1.0 mbar, for example, acetylene. Steel rings orsilicon monocrystalline discs also served as substrates.

The layers comprising 20 to 5 at % silicon and 80 to 95 at % carbon havea sliding friction coefficient against steel in an atmosphere with 50%relative air humidity of μ≈0.07 (Compare Table 4).

                  TABLE 4                                                         ______________________________________                                        Sliding layer                                                                           Carbon  Metallic element                                            (composition)                                                                           (at %)  (at %)         μ.sub.1                                                                         μ.sub.2                              ______________________________________                                        Si--C     80-95   20-5           --   0.07                                    Ta--C     95       5             0.03 0.08                                    W--C      91       9             0.06 0.14                                    Ru--C     82      18             0.03 0.05                                    Fe--C     84      16             0.14 0.14                                    ______________________________________                                         μ.sub.1 = coefficient of friction at 50% relative air humidity and a       friction member in the form of a layer of the same composition as the         sliding layer.                                                                μ.sub.2 = coefficient of friction at 50% relative air humidity and         steel as a friction member.                                              

Table 5 below indicates how the coefficient of sliding friction μ for atungsten-carbon layer containing 1.6 at % tungsten 94.5 at % carbon and3.9 at % residual gas with a friction member in the form of a layer ofthe same composition as well as with steel as a friction member isinfluenced by different relative air humidities.

                  TABLE 5                                                         ______________________________________                                        Sliding layer  Relative air                                                   (composition)  humidity (%) μ.sub.1                                                                           μ.sub.2                                 ______________________________________                                        1.6 at % W     90           0.10   0.16                                       +              50           0.08   0.12                                       94.5 at % C    10           0.06   0.07                                       +              1            0.02   0.05                                       3.9 at % residual gas                                                                        <0.4         0.01   0.02                                       ______________________________________                                         μ.sub.1 = coefficient of friction with a friction member in the form o     the same composition as the sliding layer.                                    μ.sub.2 = coefficient of friction against steel as a friction member. 

What is claimed is:
 1. A workpiece comprising a substrate having asliding surface provided with a friction-reducing coating;said coatingconsisting essentially of elemental carbon dispersed in a matrix formedof at least one metallic element in the proportions of 50.1 to 99.9 at %of the elemental carbon and 0.1 to 49.9 at % of the metallic element,the ratio of the metallic element to the elemental carbon differing fromthe stoichiometric ratio of a carbide.
 2. A workpiece according to claim1, in which the substrate is metallic.
 3. A workpiece according to claim1, in which the coating consists essentially of 60 to 97 at % ofelemental carbon and 40 to 3 at % of at least one metallic element.
 4. Aworkpiece according to claim 3, in which the coating consistsessentially of 80 to 95 at % of elemental carbon and 20 to 5 at % of atleast one metallic element.
 5. A workpiece according to claim 1, inwhich the metallic element is an element of group Ib, IIb, IIIa, IV, Vb,VIb, VIIb or VIII of the periodic table.
 6. A workpiece according toclaim 5, in which the metallic element is silicon, tantalum, tungsten,ruthenium or iron.
 7. A method of making a workpiece according to claim1, which includes coating the sliding surface of the substrate bychemical or physical vapour deposition of the elemental carbon and themetallic element.
 8. A method according to claim 7, which includesdepositing the elemental carbon and the metallic element by cathodesputtering of a target formed from carbon and the metallic element.
 9. Amethod according to claim 7, which includes depositing the elementalcarbon and the metallic element by cathode sputtering of a target formedfrom the metallic element in an atmosphere of an inert gas and ahydrocarbon gas.
 10. A method according to claim 9, which includesstarting the deposition in an atmosphere containing only the inert gasand then continuing the deposition under an inert gas-hydrocarbon gasatmosphere.
 11. A method according to claim 9, in which the inert gascomprises argon.
 12. A method according to claim 9, in which thehydrocarbon gas comprises acetylene. .Iadd.
 13. A workpiece comprising:asubstrate having a surface; and a friction-reducing coating on thesurface of the substrate having a coefficient of sliding friction;characterized in that the friction-reducing coating consists essentiallyof elemental carbon dispersed in a matrix formed by at least onemetallic element, the ratio of carbon to the metallic element rangingfrom 50.1/49.9 to 99.9/0.1, said ratio differing from the stoichiometricratio of a carbide. .Iaddend. .Iadd.
 14. A workpiece as claimed in claim13, characterized in that the friction-reducing coating is X-rayamorphous. .Iaddend. .Iadd.15. A workpiece comprising:a substrate havinga surface; and a friction-reducing coating on the surface of thesubstrate, said friction-reducing coating having a coefficient ofsliding friction; characterized in that the friction-reducing coatingconsists essentially of elemental carbon dispersed in a matrix formed byat least one metallic element, the ratio of carbon to the metallicelement ranging from 50.1/49.9 to 99.9/0.1 said ratio differing from thestoichiometric ratio of a carbide, said coating further containingimpurities in amounts which do not materially increase the coefficientof sliding friction of the coating as compared to such a coating withoutsuch impurities. .Iaddend. .Iadd.16. A workpiece as claimed in claim 15,characterized in that the friction-reducing coating is X-ray amorphous..Iaddend. .Iadd.17. A system comprising:a first workpiece having asurface; a second workpiece having a surface, the surface of the firstworkpiece bearing against and sliding with respect to the surface of thesecond workpiece; and a friction-reducing coating on the surface of thefirst workpiece; characterized in that the friction-reducing coatingconsists essentially of elemental carbon dispersed in a matrix formed byat least one metallic element, the ratio of carbon to the metallicelement ranging from 50.1/49.9 to 99.9/0.1 said ratio differing from thestoichiometric ratio of a carbide. .Iaddend. .Iadd.18. A system asclaimed in claim 17, characterized in that the friction-reducing coatingis X-ray amorphous. .Iaddend. .Iadd.19. A system as claimed in claim 18,further comprising a friction-reducing coating on the surface of thesecond workpiece.